Fetal hemoglobin (or hemoglobin F) is the major hemoglobin produced during the later part of fetal life. The genes encoding for the gamma chain (the nonalpha chain of hemoglobin F) and the beta chain (the non-alpha chain of hemoglobin A) are in close proximity of chromosome ll. At birth, hemoglobin F is replaced by hemoglobin A as the predominant protein within the red cell. However, throughout life small levels of hemoglobin F persist in a small population of cells designated as F cells. We are studying the expression of hemoglobin synthesis in cultures of erythroid cells. Depending on the source, several types of colonies designated colony forming units and burst forming units develop. Previous reports show the expression of hemoglobin F is induced in those colonies, the burst forming units, which take longer to develop and require higher levels of the hormone erythropoietin. Using a sensitive and specific radioimmunoassay we will study hemoglobin F synthesis and its relation to time, culture conditions and source of culture material and then examine the molecular mechanisms related to the switch.